Density and Pressure

Densityis the amount of matter in a volume of a substance. The symbol for density is the Greek
letter rho (r) and the formula for density is

or

The units (SI) are kg/m3. Density is also commonly expressed
in

grams/cubic
centimeter

grams/cc

grams/milliliter

These are all the same unit. The density of water is 1000 kg/m3,
or 1 g/cc at 4oC. Here are some common densities that will be useful for homework. Others can be found in the links below, or by looking them up.

Material

Density

(in kg/m3)

Material

Density

(in kg/m3)

Material

Density

(in kg/m3)

Water (at 4oC)

1000

Air

1.29

Helium

0.179

Aluminum

2700

Iron/Steel

7800

Copper

8900

Gold

19300

Lead

11300

Wood

300 - 900

Ice

917

Mercury

13600

Sea Water

1024

Specific gravityis the ratio of the density of a substance to the density of water at
4oC. A hydrometer (right) is used to measure specific gravity. For example, ethyl alcohol has a density of 790 kg/m3. It's specific gravity is 790
kg/m3/1000kg/m3 or .790.For some materials
(solids and liquids under low pressures) we can assume the density is constant under a range of pressures. Temperature can cause changes in the density of these materials. For gases, however, density
can change easily with pressure and temperature. We'll discuss this further in other lessons.